Lizhi Xiao

Theoretical investigation of heterogeneous wettability in porous media using NMR

It is highly important to understand the heterogeneous wettability properties of porous media for enhanced oil recovery (EOR). However, wettability measurements are still challenging in directly investigating the wettability of porous media. In this paper, we propose a multidimensional nuclear magnetic resonance (NMR) method and the concept of apparent contact angles to characterize the heterogeneous wettability of porous media. The apparent contact angle, which is determined by both the wetting surface coverage and the local wettability (wetting contact angles of each homogeneous wetting regions or wetting patches), is first introduced as an indicator of the heterogeneous wettability of porous media using the NMR method. For homogeneously wetting patches, the relaxation time ratio T1/T2 is employed to probe the local wettabiity of wetting patches. The T2u2009−u2009D is introduced to obtain the wetting surface coverage using the effective relaxivity. Numerical simulations are conducted to validate this method.

The downhole circumferential scanning magnetic resonance imaging tool

The downhole circumferential scanning magnetic resonance logging is able to image saturation distribution and fluid properties of stratum around a borehole, thus providing relevant and abundant information for formation evaluation. The device employs a phase-controlled excitation device based on combined array structure to accomplish three dimensional data acquisition from axial, radial and circumferential directions. This paper focuses on the design principle of device and the structure of electronic control system. A mutual coupling analysis with array antenna was carried out using inductance coupling principle, and realize the decoupling and energy discharge compensation of array antennas. The circumferential scanning nuclear magnetic resonance technique has a potential of overcoming the weakness of two dimensional measurements and raising new applications that it determines the azimuth of the fluid in the borehole and realizes the imaging measurement of the pore structure and the reservoir fluid.

Characterization of porous media by T2-T2 correlation beyond fast diffusion limit

Pore size distribution and surface relaxivity are two important properties of porous media such as rock samples and can be obtained by NMR methods. However, it is difficult to obtain these information beyond the fast diffusion limit. Here we present a new method to directly characterize the averaged pore size of a porous sample with a narrow pore size distribution. This method is based on the parallel plates pore model and the T2-T2 correlation sequence. The pore size (a) - surface relaxivity (ρ) correlation maps were obtained using the non-negative least squares method. Three kinds of glass bead samples were measured and the averaged pore size and surface relaxivity were extracted.

New magnet array design for downhole NMR azimuthal measurement

In low-field NMR, depth information and radial profile information of downhole formation can be easily acquired with the help of static gradient magnetic field produced by permanent magnets, called downhole NMR imaging. Based on the hypothesis that the formation is homogeneous, average signals detected by centralized or decentralized sensors can provide enough information for petrophysical parameters. In fact, the inhomogeneity of formation may have serious impact on description of the characteristics of formation and oil/gas location which is rarely studied in NMR well-logging. To improve this, we design and implement a new quadrupolar magnet array aimed at achieving azimuthal measurement in this paper. A new quadrupolar magnet array is consisted of four bread-shaped magnets combined with additional small hexangular magnets to produce enough strength and high homogeneity of static field along with circumferential direction at deeper DOI (depth of investigation). Azimuthal measurements are achieved by using coil array combined with quadrupolar magnet array.

A new side-looking downhole magnetic resonance imaging tool

In general, only the depth information can be acquired using the centralized downhole NMR tools. The radial profile information is equally important to the depth. Improving the pad tools, also called side-looking tools, is the appropriate direction for solving this problem. The side-looking downhole measurement can provide depth and radially resolved information of the reservoir. In this research a new side-looking tool which includes main magnets and pre-polarized magnets has been designed and built. The pre-polarized magnets in both sides are used to adjust the homogeneity of magnetic field along the length direction of the instrument and polarize the samples when the tool is moving up and down along the borehole with a speed up to 500u202fm/h. A winding coil with several frequencies corresponding to different depths has been designed to match the static magnetic field. The sensitive region of this tool is about one-third of a hollow cylinder at every frequency which gives a side-looking image of the borehole wall. We have demonstrated that this new side-looking tool behaves well with an echo time short to 0.25u202fms, which ensures the richness and accuracy of the measurements. Such a new side-looking tool is suitable for the detection of unconventional reservoirs.

NMR characterizing mixed wettability under intermediate-wet condition

Applying the concept of effective relaxivity to characterize wettability is based on the configuration of fluid distributions in porous media. However, in mixed-wet porous media with intermediate-wet patches (homogeneous wetting region), effective surface relaxivity cannot fully characterize wettability because fluid distributions are not directly corresponding to wetting patch distributions. Patches with different wettability interact with the same fluid differently, which leads to different surface relaxivity. The distribution of this kind surface relaxivity from porous media saturated with single fluid matches mixed wettability distribution. Here, we apply decay due to diffusion in internal field plus Carr-Purcell-Meiboom-Gill (DDIF-CPMG) method to obtain T2 and the pore size distribution correlation. The variation of surface relaxivity obtained from the correlation map is used to characterize wettability distributions of mixed-wet porous media. In this paper, we also redefine a parameter based on surface relaxivity distribution to evaluate mixed wettability under intermediate-wet condition. The experiment results with limestones show that, after wettability alteration of the sample, the distribution of the surface relaxivity is changed and closely correlated with pore size distribution, which demonstrates the features of mixed wettability under intermediate-wet conditions.

T1–D–T2 correlation of porous media with compressed sensing at low-field NMR

3D Laplace NMR can distinguish different components of confined fluid in sedimentary rocks, which is important to oil industry. However, the measurement time for such experiments is very long, which hinders the application in some cases such as NMR well logging. In this research, we accelerated T1-D-T2 experiment with compressed sensing (CS) method at low-field NMR. Simulation was first performed to examine the CS reconstruction method. The experiments were subsequently implemented on a 2u202fMHz spectrometer (Oxford instrument), which has a similar magnetic field strength to well logging tool. The T1, D and T2 information are obtained by the inversion recovery, pulsed field gradients and Carr-Purcell-Meiboom-Gill (CPMG) method, respectively. The subsampling is applied in T1 and D dimensions with pseudo-random sampling. The measurement time reduced from 3u202fh to 0.6u202fh with CS method and a relative error of around 5% is achieved for data with signal-to-noise ratio of 28. The water and oil peaks are clearly distinguished in the correlation maps from subsampled data. The samples with different oil-water ratio and glass bead volume fraction were measured to examine the sensitivity of this method. In addition, diffusion and relaxation properties of the correlation maps are discussed.